1. Field of the Invention
The present invention relates to a symbol reading device, and particularly to an optical symbol reading device for reading symbols on the front and back surfaces of an article moved by, for example, a conveyor.
2. Description of the Related Art
An article that is moved by, for example, a conveyor and that bears symbols to be read, movement caused by the conveyor brings the front surface of the article toward the symbol reading device and then moves the back surface away from the symbol reading device. In other words, when reading symbols on the front or back surface of an article, the input of the image data of the symbols must be carried out while the reading distance, which is the distance between the reading device and the symbols, is in a constantly changing state.
Typical methods of receiving optical image data include the use of a camera such as an image pickup tube or CCD, or directing a laser beam scan light onto the surface to which symbols are applied and detecting the reflected light.
FIG. 1 shows a symbol reading device of the prior art that reads bar code labels by scanning by a laser beam.
In the prior-art symbol reading device of FIG. 1, article detector (light projection) 26a and article detector (light reception) 26b first detect that an article bearing a bar code to be read has entered the read zone, following which image data input unit 10, which constitutes image data input section 1, directs a laser beam scanning light upon the surface of the bar code label, a photo-sensor detects the light of the laser beam scanning light that is reflected from the surface of the bar code label, an electric signal converter converts the detection lights to electric signals and these are interpreted by interpreter 2, and the interpretation results are then outputted by way of interpretation result output section 3 to an external device. Image data input section 1 that uses a camera is made up by a camera that uses an image pickup tube or CCD.
As can be understood from focusing a camera, a clear image free of blurring can be obtained in image data input by camera of the prior art by focusing such that the reading distance, which is the distance between the camera and the symbol, i.e., the article to be read, is equal to the focal length of the camera lens.
In other words, clear images that are free from blurring cannot be obtained when reading symbols on the front and back surfaces because the above-described reading distance is constantly changing as described above.
Similarly, regarding the method of the prior art in which a laser beam scan light is irradiated, it is common knowledge that the beam diameter of the laser beam scanning light such as outputted from the image data input unit is not uniform but rather, first converges with increasing distance from the image data input unit and then diverges. At the waist position at which the laser beam converges to its narrowest diameter, i.e., at the focus point, resolution is high and a clear image free of blurring can be obtained, but at positions other than the focus point, the laser beam broadens, resolution falls, and as with the above-described camera, blurring occurs and a clear image cannot be obtained.
In other words, when input of image data is carried out at points other than the focus point in a symbol reading device in which reading is realized by irradiating a laser beam upon a bar code label and then reading the reflected light, the laser beam diameter is broad and resolution is low. As a result, the bar widths that make up a bar code cannot be accurately detected and the reading performance of the symbol reading device suffers. In image data input by means of a camera as well, the image goes out of focus, clear image data cannot be obtained, and reading performance drops.
For these reasons, large symbols have been used with symbol reading devices of the prior art that read symbols on front and back surfaces, thereby reducing the effect of the above-described blurring.
With large symbols, symbol data are not lost despite blurring. Larger symbols, however, require larger labels or paper for printing, and therefore entail the disadvantage of increased operation costs.
A further disadvantage is that larger labels cannot be applied to small articles.
It is an object of the present invention to provide a high-performance optical symbol reading device that is capable of always matching the focus point to the front or back surface of an article when reading the front surface or back surface of an article for which the reading distance is in constant change.
The optical symbol reading device of this invention is made up of: an image data input section that is provided with an image data input unit for receiving a bar code label, characters, symbols, or image data on an article that is moved by a conveyor; an article detector for detecting that an article has entered the read zone; an interpreter for converting electric signals from the image data input section to numbers or characters; and an interpretation result output section for outputting the interpretation results of the interpreter to an external device;
wherein the image data input section is provided with:
an image data input focus point modifier;
and the optical symbol reading device further comprises:
a front surface/back surface position detector for detecting the position on the conveyor of the front surface or back surface of an article that is moved by the conveyor, and an image data input focus point control section that outputs information from the front surface/back surface position detector to the image data input focus point modifier.
In addition, the front surface/back surface position detector is provided with means that is provided with a light projection position detector and a light reception position detector made up of a plurality of transmissive multiple optical axis sensors, for finding the position of the front surface or back surface of an article by detecting which transmissive multiple optical axis sensors of the plurality of transmissive multiple optical axis sensors of the light projection position detector are being shielded by the article.
In addition, the front surface/back surface position detector is provided with means that includes a rotary encoder that is attached to the conveyor, for finding the position of the front surface or back surface of the article by counting pulses from the rotary encoder and measuring the distance of movement of the conveyor.
In addition, the image data input focus point control section may also include means for converting the front surface/back surface position data of the article that are received from the front surface/back surface position detector to a reading distance, which is the distance between the image data input unit and the front surface or back surface of the article, and outputting the reading distance as focus point data to the image data input focus point modifier.
In addition, the image data input focus point modifier may include means for matching the focus point to the front surface or back surface of an article that is constantly moving over time by setting the focus point to a position designated by focus point data that are received from the image data input focus point control section.
Further, means may be included for reading two surfaces, i.e., the side surface/back surface or side surface/front surface of an article that is moved by a conveyor, by fixing the focus on the position of the side surface and reading the side surface when receiving a bar code label, characters, symbols, or image data on a side surface of the article from the image data input unit.
In other words, this invention provides an image data input focus point control section that outputs position data of an article that are received from the front surface/back surface position detector to an image data input focus point modifier for the purpose of obtaining clear image data of, for example, symbols applied to the front or back surface of an article that is being moved by, for example, a conveyor and for which the reading distance is constantly changing, thereby enabling matching of the focus point of a camera or waist point of a laser beam for image data input to the front or back surface of the article and realizing the input of clear image data.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate an example of a preferred embodiment of the present invention.